Fabrication process and package design for use in a micro-machined seismometer or other device

ABSTRACT

An accelerometer or a seismometer using an in-plane suspension geometry having a suspension plate and at least one fixed capacitive plate. The suspension plate is formed from a single piece and includes an external frame, a pair of flexural elements, and an integrated proof mass between the flexures. The flexural elements allow the proof mass to move in the sensitive direction in the plane of suspension while restricting movement in all off-axis directions. Off-axis motion of the proof mass is minimized by the use of intermediate frames disbursed within and between the flexural elements. Intermediate frames can include motion stops to prevent further relative motion during overload conditions. The device can also include a dampening structure, such as a spring or gas structure that includes a trapezoidal piston and corresponding cylinder, to provide damping during non-powered states. The capacitive plate is made of insulating material. A new method of soldering the capacitive plate to the suspension plate is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of a prior application Ser. No.11/414,866 entitled “FABRICATION PROCESS AND PACKAGE DESIGN FOR USE IN AMICRO-MACHINED SEISMOMETER OR OTHER DEVICE” filed on May 1, 2006, whichis a continuation-in-part of application Ser. No. 10/851,029 entitled“IMPROVED MICRO-MACHINED SUSPENSION PLATE WITH INTEGRAL PROOF MASS FORUSE IN A SEISMOMETER OR OTHER DEVICE” filed on May 21, 2004, now U.S.Pat. No. 7,036,374, which is a continuation-in-part of application Ser.No. 10/058,210, filed on Jan. 25, 2002, now U.S. Pat. No. 6,776,042,which are hereby incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to seismic instrumentation in general. Moreparticularly, the invention is related to improved fabrication andpackaging techniques for a micro-machined suspension plate having anintegral proof mass and a method of fabrication for the same that may beutilized in a seismometer (velocimeter), an accelerometer, or othersimilar device.

2. Description of the Prior Art

U.S. Pat. No. 6,776,042 discloses a novel construction of anaccelerometer or seismometer using an in-plane suspension geometryhaving a suspension plate and at least one fixed capacitive plate. Incontrast to conventional seismometers, which utilize a spring supportinga distinct proof mass on an external frame, the micro-machinedsuspension plate is formed from a single piece of material to includethe external frame, a pair of flexural elements and an integral proofmass interposed between the flexures. The flexural elements allow theproof mass to move in one direction, the sensitive direction, in theplane of suspension, while restricting as far as possible movement inall the other off-axis directions.

The new in-plane design also includes a displacement transducer fordetermining relative motion of the proof mass. This transducer includesaccurately placed drive electrodes, preferably positioned on the proofmass, and corresponding pickup electrodes located on the fixedcapacitive plate.

The device further includes either electrostatic or electro-magneticactuators that can be used within an electronic control loop tore-center the proof mass by exerting a restoring force in a so calledforce re-balance control system.

U.S. patent application Ser. No. 10/851,029 entitled “IMPROVEDMICRO-MACHINED SUSPENSION PLATE WITH INTEGRAL PROOF MASS FOR USE IN ASEISMOMETER OR OTHER DEVICE” discloses improvements to the basic designincluding adding intermediate frames disbursed within and between theflexural elements in order to produce a system where the frequency ofthe off-axis modes are as many multiples as possible of the resonantfrequency of the system, while minimizing the reduction in thefrequencies of spurious modes along the sensitive axis.

The intermediate frames are provided with motion stops, so that underoverload conditions the frames engage each other, preventing furtherrelative motion, before the flexures make any contact or becomeoverstressed. These stops thus minimize the chance of fracture or theirreversible surface bonding of portions of the flexure (“stiction”).

The structure includes a dampening structure that is highly effectiveduring non-powered/non-operational states (i.e. when the feedbackcontrol system is not powered and does not provide any dampening). Thisdampening structure includes a spring/gas dampening structure configuredto provide damping during non-powered states.

For the packaging of such a device, it is well known that melted solderballs can be used to form conducting interconnects off a microelectronicdevice (chip) and between chips. This knowledge is widely used to bond,and form electrical connections between, a chip and a substrate. Thesubstrate generally provides conductive paths to and from the chip, orchips, bonded in this way to it. This technique has been extended to thepackaging of micro-electro-mechanical (MEMS) devices (U.S. Pat. Nos.6,808,955, 6,852,926, 6,903,452, and 7,030,432) in which as well asproducing electrical and mechanical interconnects, the solder balls aredisposed between two wetting rings on separate MEMS substrates, so thatreflow of the solder balls forms a contiguous ring producing a sealedcavity. A design that uses solder balls to both form a seal and controlthe spacing of the substrates, has not been presented.

The use of apertures through a frame to position solder balls onto asubstrate is known (U.S. Pat. No. 6,857,183). The use of a carrier tohold solder balls within recesses using a vacuum force is also known(U.S. Pat. No. 6,916,731). Controlling the depth of the frame, orrecess, to allow only one solder ball to occupy the desired position hasbeen explained in both U.S. Pat. Nos. 6,857,183 and 6,916,731. Neitherof these techniques has been extended to the scale of the currentinvention. Furthermore, there is no prior art for self-aligning thesolder balls directly on the substrate to be bonded.

The use of thin micro machined bridges to support MEMS structures whilethermally isolating them from their environment is known (U.S. Pat. No.6,900,702). However, the design for a device that provides maximumthermal isolation utilizing a minimum of substrate area, and maximizingmechanical rigidity, has not been presented.

SUMMARY OF THE INVENTION

The present invention is a further improvement upon the invention of theU.S. application Ser. No. 10/851,029, describing an off-axis motion ofthe proof mass is minimized through the use of intermediate frames. The'029 application, in turn, improves upon the design set forth in U.S.Pat. No. 6,776,042 by utilizing intermediate frames. The number offrames to be used is determined as a function of both the desiredbandwidth over which spurious modes are to be eliminated and the desiredoperational parameters of the system. More particularly, as the numberof frames is increased, the off-axis spurious resonant modes are pushedup in frequency, thus increasing the overall effective bandwidth overwhich the device may operate without the occurrence of any spuriousresonant frequencies. However, as the number of frames is increased, thefrequency of spurious modes along the sensitive axis is reduced, due tothe additional mass of the frames. Accordingly, a balance is struckbetween the desired elimination of off-axis and on-axis spuriousresonant frequencies over an operational.

The intermediate frames can be provided with motion stops, so that underoverload conditions the frames engage each other, preventing furtherrelative motion, before the flexures make any contact or becomeoverstressed. These stops thus minimize the chance of fracture or theirreversible surface bonding of portions of the flexure (“stiction”).

The invention also preferably includes a dampening structure. In apreferred embodiment, the structure preferably includes a trapezoidalshaped piston and a corresponding engagement cylinder. The dampingstructure is positioned to engage between the outermost intermediateframe and the external frame as the springs are overloaded. In this way,and as explained earlier, the piston or cylinder is placed at atraversal distance which extends further than any intermediate flexuralelements such that it will not make contact with any of these flexuralelements. The piston or cylinder faces outward, and a correspondingcylinder or piston is then positioned on the inner surface of the outerframe of the suspension plate, facing inward toward the proof mass.

As the most outward intermediate frame approaches the inner surface ofthe outer frame of the suspension plate, the piston will engage thecylinder, thereby providing a dampening effect before the intermediateframe can contact the surface of the external frame of the suspension.In a preferred embodiment where the suspension plate is contained andsubmersed within a gaseous environment, the pressure of the gas willincrease within the confined space of the cylinder as the piston movesfurther into the cylinder. The resulting viscous gas flow will act as adamping force, slowing the outer intermediate frame away from theexternal frame of the suspension. In an alternative embodiment where nogas is used, the piston and cylinder may be coupled using a dissipativematerial disposed between the piston and the cylinder such that thematerial is compressed as the piston moves further into the cylinder,thereby providing a damping force which slows the motion of the outerintermediate frame toward the external frame.

As a further improvement, the present invention involves a technique ofbiasing the springs by the geometrical arrangement of the mask to ensurethat the proof mass is centered when the device is orientated at the socalled Galperin angle of 54.7 degrees to the vertical. Thisconfiguration allows the construction of an orthogonal tri-axial sensorarray utilizing identical spring mass systems, this is a particularlyimportant factor for economically producing these devices as batchfabricated devices on a single wafer as only one configuration ofdevices is required to produce a sensor capable of sensing the vectorfield of the velocity or acceleration.

An additional fabrication improvement is to utilize an insulatingmaterial such as glass to produce the capacitive plate(s). Byconstructing the displacement transducer pick-up capacitor as asymmetric differential metal pattern and using an insulator rather thana semiconductor as the substrate the effects of stray capacitances canbe significantly reduced and the remaining stray elements are balancedbetween the two capacitive pick-up arrays. Using the same thicknessinsulator for either two capacitive plates or one capacitive plate and abacking plate ensures no bending moments are generated as a result ofmismatches in thermal expansion between the capacitive plates and thesuspension plate.

The fabrication of three-dimensional shapes in insulators such as glassis not economical using the DRIE methods used to form the suspensionplate. However, the techniques of micro-abrasion using a stream ofabrasive particles carried in a high velocity gas jet can be used tocreate the features required in brittle materials such as glass andceramics.

In order to allow wafer level fabrication of a sandwich of capacitiveplate, suspension plate, and capacitive plate or backing plate it isnecessary that the die sandwich be separated easily. This can beaccomplished by using a dicing saw, but such a process requires “tramlines” that waste wafer area, requires continuous straight line cuts,and causes chips and debris that need to be carefully removed from thefinished devices. The silicon suspension plate wafer can easily bedesigned so that the individual die are easy to singulate as only smallrelatively weak tabs are left between the individual die after the DRIEprocess. This technique cannot be used on the insulators of thecapacitive and backing plates which often as in the case of glass tendto suffer from conchoidal fractures as there are no distinct crystalplanes for fractures to propagate. To overcome this problem we havedeveloped a technique of creating perforated weak areas between the diethat direct the cracks in the correct direction allowing the twoinsulating wafers to break cleanly along with the silicon suspensionplate.

The backing wafer can be attached to the silicon proof mass wafer usinga variety of techniques known to those skilled in the art, such as glassfrit bonding, anodic bonding, eutectic solder bonding.

However the operation of the displacement transducer and the electricalinterconnection of the seismometer require that the capacitive plate andsuspension plate be separated by a well-controlled distance of the orderof 40 microns for a normal seismometer with a sub nano-g resolution. Theseparation must also allow conductive paths to be formed between the twoplates. The innovative solution for this is to use the re-melting andflowing together of precisely arrayed solder balls of very uniformdiameter to form a closely dimensioned seal between the two wafers to bebonded. The flow path of the solder balls is controlled by a patternedwetting metal layer on each of the wafers to be bonded, a well-knowntechnique. The invention here consists of controlling the disposition ofthe solder balls and their application to form a precisely dimensionedseal of well controlled thickness.

Solder sealing also can be utilized to vacuum-package the mechanicalelements of the device. Without vacuum packaging the performance islimited by the damping of the motion of the mechanical elements by air:for a 20 mm×20 mmm square 0.5 mm thick silicon suspension plate with thehollow cavity filled with gas at atmospheric pressure results in adevice with a “Q” of the order of several hundred, a mass of the orderof 0.25 g and a resonant frequency of approximately 10 Hz, and that canprovide sub nano-g resolution based on the “MTQ” product. For very quietsites seismometers are required with resolutions down to the 10's ofpico-g levels. The only parameter that can be changed significantly fora practical MEMS device is the “Q”. We have demonstrated devices with aQ of 40,000 in vacuum. The ratio of this to our “Q” in air of 400 is100, which accounting for the square root relation of MTQ product toacceleration noise would result in a theoretical resolution of betterthan 10⁻¹⁰ g's.

The solder sealing technique we have proposed or other techniques can beused to produce a vacuum seal and the use of gettering technology inMEMS devices to produce vacuum levels of milli-torrs or less is welldocumented in the literature and provided as for example by thecommercial product “Nano-getters”. A novel feature of the seismometerdesign is that as the capacitive plate is formed of glass optical powercan be coupled into the device from a laser, this allows the activationof the getter with out resisitive heating elements, and withoutremelting the solder seal.

However, several other performance parameters are also required toimprove to achieve these extremely high sensitivities. The displacementtransducer noise floor needs to be improved and this can be done bydecreasing the spacing between the suspension plate drive capacitors andthe displacement transducer pick-up capacitors on the capacitive plateand decreasing the period of the displacement transducer arrays.

The completed seismometer dies needs to be mounted so they aremaintained in the correct geometric orientation to each other and thereference surface of the instrument. The devices are packaged by theformation of the sandwich of three parts, which encases the proof massand flexural elements within the package. However, it is important forthe correct operation of the unit that the complete package is notsubject to stress caused by thermal mismatch of the mounting, which isnormally metallic, to the insulator silicon sandwich of the seismometerdie. Using abrasive machining a novel mounting can be formed as part ofthe normal capacitive plate fabrication that ensures a sufficientlyconstrained, but not over-constrained mounting minimizing thetransmission of thermal stresses to the seismometer die. The devicemounts using three precision metal or ceramic balls. The first ballmounts in a machined hole, the second in a machined slot, and the finalball on the flat surface of the device. A resilient foam pad pushes thedevice down onto these mounting balls. This forms an ideal three-pointmounting for the device as the first ball locates the device at a pointin space, the second ball locates it on a defined line direction, whilethe third ball constrains it to a known plane in space.

The seismometer die requires electrical connections to the electronicpre-amplifiers mounted on the printed circuit board that introduce theminimum of capacitive strays and do not cause those strays to vary overtime or temperature. The connections thus need to be as short aspossible. Thus, wire bonded wires with their required service loop andtendency to move under shock or vibration are not ideal. The device usesa novel method of using an elastomeric connector preferably withembedded gold-plated wires as the connection path. By using the accurateDRIE and abrasive machining process the holder for this device can bemade integral to the overall die as part of the standard fabricationprocess, reducing costs. The elastomeric connector also works in concertwith the resilient foam retaining the seismometer die and does not causeadditional stress on the die.

A further problem limiting reductions to the noise floor is the thermal,rather than, inertial response of the device. The silicon flexuralelements are under a gravity bias of ˜0.6 g in the Galperinconfiguration. Silicon has a temperature coefficient of elasticity of˜120 ppm/C. As the temperature increases the flexural elements weakenand the proof mass moves creating the same signal as an actualacceleration. In the most accurate mechanical seismometers this effectis compensated by using a spring with a close to zero temperaturecoefficient of elasticity as the flexural element. Although we cannotfabricate a compensated spring using current silicon technology we canisolate the flexural elements from the temperature variations so thatthe temperature induced error is outside the lowest frequency bandwidthof interest. This requires us to substantially attenuate the amplitudeof the temperature variations down to periods of 20 seconds to be of theorder of a few micro Kelvin.

We can much improve thermal isolation by reducing conduction from thedevice portion of the suspension plate to the external environment withthe introduction of a micro machined sinuous thermal pathway whichutilizes a minimum die area. This thermal frame is produced bythrough-wafer etching, preferably using DRIE, and can be formed at thesame time as the proof mass and flexural elements of the seismometer.

The thermal frame construction and the vacuum packaging reduce greatlyreduce the thermal conduction losses and eliminate convection losses.However, radiative losses can still be significant. These can be reducedby using low emissivity layers on the glass capacitive plate and backingplates to minimize radiative loss. These can be applied as metalliclayers during the fabrication of these plates.

Further improvements can be made by using two sets of thermal frames andan internal glass cover set to provide a double radiative barrier, and athermal reservoir within the device. Obviously this feature can beextended to as many internal covers as required in a “Russian Doll”concept.

These techniques are all passive. Active power dissipation in theelectro-magnetic feedback coils can cause temperature fluctuations dueto the resistive heating on the proof mass and on the flexural elementsthemselves. Using an additional feedback circuit and an AC signaloutside of the band of interest the total power dissipation can be keptconstant removing this particular error source. This constant powercircuitry is a novel solution to this particular problem.

Finally the system can be modified further by measuring the temperaturevariations externally to the device the temperature of the inner framecan be actively controlled by using a control loop (preferably digitaldue to the extremely long periods required) and heat generatingresistors on the frame to prevent the flexural elements seeing theexternal temperature variations. These packaging techniques and thermalcontrol methodologies, both passive and active have generalapplicability to other MEMS sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross section of a seismometer die and magneticcircuit identifying the various elements of the device.

FIG. 2 shows the components of the suspension plate. It shows theflexural elements and intermediate frames.

FIG. 3 shows a mask set that has been deliberately biased so that theflexural elements and intermediate frames are “pre-tensioned” when lyingflat.

FIG. 4 shows the preferred embodiment of a capacitive plate fabricatedby micro-abrasion.

FIG. 5 shows the fabrication of a perforated support between capacitiveplates using micro-abrasion.

FIG. 6 shows the fabrication of a controlled solder seal using precisionsolder balls and a carrier wafer.

FIG. 7 shows the fabrication of a controlled solder seal using precisionsolder balls without the use of a carrier wafer.

FIG. 8 shows the controlled orientation minimum stress mounting of theseismometer die using precision metallic or ceramic balls.

FIG. 9 shows the use of an elastomeric connector to connect theseismometer dies to the traces on a printed wiring board.

FIG. 10 shows the design of the thermal isolation structure.

FIG. 11 shows the design of the radiative shields and multipleenclosures.

FIG. 12 shows a block diagram of the proposed active control systems forreducing the effect of coil heat dissipation and actively controllingthe inner enclosure or frame.

FIG. 13 illustrates the spurious mode rejection ratio for in-axis andout-of axis modes as the number of intermediate frames is increased in apreferred embodiment having six flexural elements on each side of theproof mass.

FIG. 14 illustrates the spurious mode rejection ratio for in-axis andout-of axis modes as the number of intermediate frames is increased in apreferred embodiment having twenty-four flexural elements on each sideof the proof mass.

FIG. 15 illustrates a perspective view of a suspension plate having aspring/gas dampening structure in accordance with a preferred embodimentof the present invention.

FIG. 16 illustrates a close-up view of a preferred embodiment of thespring/gas dampening structure.

FIG. 17 illustrates a close-up view of a piston used in an alternativeembodiment of the spring/gas damping structure.

FIG. 18 illustrates another close-up view of an alternative piston usedin an alternative embodiment of the spring/gas damping structure.

DETAILED DESCRIPTION OF THE INVENTION

As explained earlier, U.S. Pat. No. 6,776,042 entitled “MICRO-MACHINEDACCELEROMETER” discloses an improved micro-machined suspension platewhich may be utilized in an accelerometer, seismometer (velocimeter)and/or other similar device. The subsequent U.S. patent application Ser.No. 10/851,029 entitled “IMPROVED MICRO-MACHINED SUSPENSION PLATE WITHINTEGRAL PROOF MASS FOR USE IN A SEISMOMETER OR OTHER DEVICE” disclosesimprovements to the basic design of the suspension plate.

The suspension plate of the '029 Application is formed of and includes arevolutionary, in-plane suspension geometry rather than atraditional—spring design. More particularly, the suspension plate ismicro-machined to form a central proof mass and flexural elementslocated on opposite sides of the proof mass. FIG. 1 illustrates across-sectional diagram of a seismometer 1 having a suspension plate 2and two capacitive plates 3 a-b (alternatively, the device can have onecapacitive plate), with a centrally located proof mass 8 supported byflexural elements 6 utilized in a known, prior-art micro-machinedin-plane suspension geometry, as described and set forth in U.S. Pat.No. 6,776,042.

As shown in FIG. 1, the proof mass 8 is centrally located and surroundedby a hollow cavity 4. The flexural elements 6 extend from oppositedirections and allow the proof mass 8 to move in one direction, in theplane of suspension, but suppress motion of the proof mass in all otherdirections. These flexural elements 6 represent a significantimprovement over the conventional use of a mechanical cantileveredspring design for supporting the proof mass.

FIG. 2 illustrates a suspension plate having a proof mass 201 supportedby flexural elements 202 and further having intermediate frames 204inter-disposed there between, in accordance with a first preferredembodiment of the present invention. Use of these intermediate frames204 eliminates any spurious modes over a much larger bandwidth andallows the production of a device with a flat response over the regionof such bandwidth. The intermediate frames 204 also provide additionalsupport to the proof mass 201 and help reduce the out of plane sag. Thisimprovement was disclosed in U.S. patent application Ser. No.10/851,029.

For practical production of a seismometer device having a suspensionplate and two conductive or capacitive plates, as described in U.S. Pat.No. 6,776,042, it is highly desirable that a single device geometry canbe used to produce all three components of the sensor—i.e. thecapacitive plates and the suspension plate. In order to accomplish this,all three plates are preferably arranged in a “Galperin” orientation soeach sees the same gravity vector. Due to the geometry of the device itis important to ensure for optimal operation and design that whenexposed to this gravity vector the proof mass is centered. If thesuspension plate it manufactured separate from the capacitive plates,then the gravity force on the proof mass will effect the centering ofthe proof mass relative to each of the other capacitive plates and thiswill affect the readings as to each plate when the whole device isformed.

To ensure that the proof mass is centered after production, the mask setis deliberately biased so that the flexural elements are “pre-deflected”when lying flat. This pre-deflection is such that when orientated at the“Galperin” orientation or angle of 54.7 degrees, to the vertical thespring mass system is centered. When the material is removed by a methodsuch as Deep Reactive Ion Etching (DRIE) the spring assumes a centeredposition at the Galperin angle of 54.7 degrees. The pre-deflection canbe calculated either analytically or using Finite Element Analysis, bothtechniques are well know to those skilled in the art, such that thepattern is the same deflection pattern that would be observed in areleased symmetrical structure when subject to an acceleration ofopposite magnitude and direction to that the system when orientated atthe Galperin position. This level of pre-deflection will then almostexactly counterbalance deflection due to the gravity vector in theGalperin orientation so that the mass will be nearly perfectly centered.FIG. 3 illustrates a mask set that has been deliberately biased so thatthe flexural elements are “pre-deflected” when lying flat.

FIG. 4 shows the design of a capacitive plate 400 using an insulatorsuch as glass. The use of an insulator rather than a semiconductor forthis plate ensures that the stray capacitance from the displacementtransducer pick-up capacitor 402 is minimized as stray capacitance tothe semi-conductor substrate is eliminated. A further improvement isrealized by using a differential pick-up capacitor such that common modepick up of extraneous signals can be rejected in the electronics. Thetwo capacitors, shown as 403 and 404 in the enlargement, illustrate thegeometrical design for such a pickup array. The capacitors driving thisarray are placed on the proof mass and are a similar pattern ofinter-digitated fingers with the same repeat period as the capacitivepick up array. One problem of insulators such as glass is that they aresubject to surface charge build up which can adversely affect thedevice. To prevent this very high resistivity film 410 such as indiumtin oxide can be applied over the surface to prevent charge build up.

An important feature of the design is that whether two capacitive platesare used or one capacitive plate and a backing plate these plates shouldbe of the same thickness to ensure that the overall seismometer diesdoes not bend clue to thermal mismatch between the capacitive plate(s)and the silicon suspension plate.

The cross section 401 of the capacitive plate shows such a capacitiveplate being formed by micro-abrasion from both sides of the plate usinga protective mask. The metallization pattern is first applied to theplain wafer to form the displacement transducer pickup capacitor 402 theinterconnection paths and the connection pads 408. The metal is thenprotected with the masking material. The first abrade then forms thecontrolled depth hollow 406 and the structure including the support beam412 and the pedestal for the displacement transducer pickup capacitor402. The depth can be controlled by careful control of themicro-abrasion parameters, particle size, gas pressure, nozzle diameterand distance from the work piece, and running for a constant time withthe nozzles moving at a constant velocity across the part. The secondabrade them forms trenches to allow the individual capacitive plates tobe separated and the structures for the mechanical support of theseismometers. Through wafer tooling holes are also formed to allowmechanical alignment of all elements of the seismometer.

To allow the seismometers to be assembled at the wafer level it isimportant that the capacitive plate, suspension plate and the backingplate remain as a contiguous wafer until they are bonded together.Separating these by a dicing saw is not a good process as explainedearlier. In FIG. 5 we illustrate in cross section the process and maskrequired to form perforated support areas that will fracture in acontrolled manner to allow the devices to be separated. In FIG. 5 a themetalized capacitive plate wafer 500 has been masked with a suitableabrasion resistant cavity mask 502. The technique for forming such amask is known to one skilled in the art of abrasive machining. In FIG. 5b the abrasive jet 503 has created a cavity trench on the top surface.In FIGS. 5 c and 5 d the bottom mask 506 also illustrated in the figurehas been applied to the bottom surface and the abrasive jet 503 has beenapplied to the bottom surface. In FIG. 5 c the abrasive jet has cutthrough the wafer completely to form a through trench 508, while in FIG.5 d the abrasive cut has created a pit shaped perforation 510. When thedevices are singulated the fracture line 512 will be directed by theweak area of the pits to follow the desired path and not damage thedevice on either side.

The glass backing wafer can be attached to the silicon proof mass waferusing a variety of techniques known to those skilled in the art, such asglass frit bonding, anodic bonding, eutectic solder bonding.

Solder balls can be aligned on one of the wafers to be bonded bydepositing a volume of solder in molten form through a positionablemicrojet, using precise “pick and place” machinery, or by deposition viaholes in a solder-ball frame. The solder balls in the latter two casesare immobilized on the wafer to be bonded by a partial re-melt beforethe second wafer to be bonded is aligned to the solder-ball carryingwafer, and full reflow performed. Our technique is an extension of thethird, whereby the solder-ball carrier is formed by micromachining asilicon wafer, preferably by DRIE, with an array of circular holes in amirror image of the final solder-ball locations on one of the wafers tobe bonded. This wafer we call the solder-ball alignment wafer 601. Inthe alignment wafer, the diameter 603 of the solder ball holes 602 is alittle larger than the solder balls 605, and the depth 604 of the holesa little less than the diameter of the solder balls. In one example, thehole dimensions for 100-micron-diameter solder balls was 105-micronsdiameter and 90-microns depth. As solder balls are available withtolerances of 2 microns in their diameter, lateral positioning can beperformed to very nearly as tight a tolerance, as the hole diameter needonly be slightly larger. An excess of solder balls 606 used to populateall the holes required for sealing either a die or a wafer is pouredonto the micro machined solder-ball alignment wafer (FIG. 6 b), which isthen gently vibrated by hand to ensure all the holes are populated. Thedimensions of the holes 602 ensures a single solder ball 605 occupieseach one, and the excess solder balls 606 can be poured off by slightlytilting the carrier wafer and reused (FIG. 6 c). To improve location andretention of the solder balls, which may be deflected by electrostaticforces, a further design modification to the solder ball wafer carrieris the inclusion of through wafer vertical channels from the un-recessedsurface of the wafer to allow the application of a vacuum. The channelsshould be of a smaller diameter than the solder balls so that areasonable seal is produced once the recess is occupied by a solderball. The channels may be produced by DRIE from the lower surface of thewafer. The wafer to be bonded 607 is then offered face down to thealignment wafer for alignment between the solder balls and the patternedwetting layer 608. This inter-wafer alignment can be achieved eitherthrough visual manipulation, if the wafer to be bonded is transparent,through infrared (IR) imaging assisted manipulation, if the wafer is IRtransparent, or by using alignment holes in both wafers with eitherprecisely dimensioned rods or balls to mechanically lock the two wafers.After alignment the solder balls are immobilized on the wafer to bebonded either with a partial reflow onto the wetting metal layer, or byadhesion to a thin film of solder flux which has been previouslydeposited on the wafer to be bonded. After the solder balls are thusimmobilized, the alignment wafer can be removed (FIG. 6 e), a procedurewhich will not be impeded by any reflow as the solder will not adhere,but rather de-wet the silicon of the alignment wafer. If vacuum has beenused to hold down the solder balls, it should be during this stage ofthe process. The second wafer to be bonded 609 with its wetting pattern610 can then be aligned to the solder-ball carrying wafer to be bondedand the final bond achieved (FIG. 6 f) through heating and reflow of thesolder 611. The thickness control of the seal is achieved by knowing theexact volume of solder in the solder balls 605 and the exact pattern ofthe metallization on both wafers 608 610 by controlling these parametersthe solder reflow 611 will result in a controlled separation 612 betweenthe wafers.

When one of the wafers to be bonded has a flat surface, an extension ofthis technique can be performed without the need for an alignment wafer.The initially flat wafer to be bonded 700 is in this case patterned withthe solder ball holes 701. Subsequent populating of the holes andalignment to the other wafer to be bonded 702 is as before, (FIGS. 2b-d) but a full reflow is then performed (FIG. 2 e). The pattern of thewetting metal 703 around the alignment holes on the flat wafer to bebonded is such that reflow de-wets the solder balls from the solder ballholes and then re-wets the metallization on the second wafer 704 andthen the metallization on the first wafer 703, forming the reflowedsolder bond 705 between the two wafers with controlled separation 706.

To ensure a precise alignment of the seismometer die to the mounting athree point mounting technique is used that precisely constrains butdoes not over constrain the seismometer die. This technique has generalapplicability to MEMS devices that need to be accurately mounted withminimal thermal stress. The capacitive plate 800 has a precisiondiameter hole abraded into it 802, and a slot with the same minordiameter 804, and a smooth un-machined surface 806 is available. Tomount the device it is located at a point in space by a precisionmetallic or ceramic ball 808 located in the hole 802, a second ball 810aligns the die along a line between the hole 802 and the slot 804.Finally the third ball 812 defines a point in space on the die 806fixing its location in space. The force 814 from a resilient pad thenpresses on the die keeping it located onto the three point supportprovided by the balls.

The use of an elastomeric connector that preferably uses embedded goldplated wires allows for minimum capacitance, minimum stress electricalconnections between the seismometer die and the electronics. In FIG. 9the suspension plate 900 has a slot etched 901 etched through it duringthe DRIE process needed to form the other structures. The dimensions ofthis slot 902 are designed such that it ensures the correct degree ofcompression on the elastomeric connector 902 as this is sandwichedbetween the seismometer die and the printed circuit board 914. Theelastomeric connector 902 makes contact with the electrical connectionpads 908 on the capacitive plate 904 using the e embedded gold platedwired 910. These wires 910 then make electrical connection to theprinted circuit board traces 912 on the printed circuit board 914. Thebacking plate 906 is machined to clear the printed circuit board. Thedesign of the elastomeric connector 902, contacts 908, and traces 912 issuch that the pitch of the gold wires 910 is designed that no pads 908can be connected to the wrong trace 912 by the wires 910.

The preferred design for thermal isolation by through-wafer etching isillustrated in the plan view of a micro machined die in FIG. 10. Thecentral portion of the die 1000 can be used to fabricate any sensorstructure which would benefit from thermal isolation. The conductivethermal coupling is reduced by etching out much of the die towards theedge to leave a series of thin beams 1001 and interconnections at themidpoints 1002 and corners 1003. In incorporating a thermal frame intoan inertial sensor, it is important not to compromise the dynamics ofthe coupling between the sensor on the central portion of the die andthe environment. Hence the design has to ensure maximum rigidity at thesame time as producing the longest thermal path from the frame to thecentral die. For a vertical downwards acceleration, the central die issupported by the sets of beams on the left and right hand side, withinwhich there will be compressive and extensive strains. The centralinterconnections 1002 have no overall stress at these, the weakest,points. The upper and lower sets of beams take very little of theload—and would not be very rigid if they did as they would undergocantilever deflection. Without the left and right beam sets the thermalframe produces a non-rigid suspension geometry. The external frame 1004forms the connection to the external packaging of the die.

The structure will be very rigid below Euler's critical loading of thecompressed beams with no bending of the beams. Above that loading theside beams will deflect as cantilevers until the beams touch, at whichpoint the structure will become rigid again. From the formula for thecritical loading, F_(crit),

$F_{crit} = \frac{\pi^{2}{EI}}{L^{2}}$

where E is Young's modulus, I is the second moment of the beam, whichfor a rectangular cross-sectional beam as produced by DRIE is w³ t/12,where w is the width of the beam, t is the thickness of the wafer, and Lis the length of the beam, approximately half the die size. Theacceleration to reach critical loading can then be calculated to be

$a_{crit} = \frac{\pi^{2}{Ew}^{3}}{24\rho \; L^{4}}$

where ρ is the density of silicon. For a 2 cm die and 40-micron beams,a_(crit) is about 5 g. Below 5 g, the resonant frequency of thisstructure is approximately 5 kHz. The dynamics of the structure could beexploited for shock protection.

The thermal behavior can be simply modeled. The structure above has twoperiods of thermal isolation structure. For each period there are eightequivalent thermal paths of length 2 L. The thermal conductance istherefore given by:

$Y = \frac{8\kappa \; {wt}}{2\; {LN}}$

where κ is the thermal conductivity of silicon, and N is the number ofperiods of thermal isolation structure. The structure implemented byDRIE would in fact have parallel beams, approximately spaced by w, andso if a border width on each side of the die, x, is given to thermalisolation, N=x/4w, and so

$Y = \frac{16\kappa \; w^{2}t}{Lx}$

The thermal capacity of the central die, treating it as an un-machinedblock of silicon, is given by

C=4L²tρG

where G is the heat capacitance of silicon. The thermal time constantnow becomes

$\begin{matrix}{t = {C/Y}} \\{= \frac{L^{3}\rho \; {Gx}}{8\kappa \; w^{2}}}\end{matrix}$

For a 2-cm die, a 1-mm margin and 40-micron beams t is 30 minutes. Theconductance is 0.05 mW/K. If 2 mm is set aside and 20-micron beams andspacing are achievable, a four-hour time constant is obtained and theconductance is reduced to 0.006 mW/K and only 0.5 milli Watts would berequired to hot bias the sensing element by 80 degrees Celsius.

In addition, the suspension itself further reduces the thermalconductance by a small amount. For 30-micron springs, with effectivelyhalf the thermal pathways and four thermal periods per spring set (eightcantilevers), they have an additional 100-s period per spring set.

All the above considers just conductive losses. Effective radiativeconductance will be given from Stefan's law as approximately

Y_(rad)=4εσL²T³

where ε is the emissivity of the die, s is Stefan's constant and T isthe temperature. This gives the ratio of radiative to conductive lossesas:

$\frac{Y_{rad}}{Y_{cond}} = {\frac{ɛ\sigma}{\kappa}\frac{L^{3}T^{3}}{w^{2}t}}$

For ε of 0.01, for a 2-cm die with 1-mm thermal margin and 40-micronbeams, Y_(rad)/Y_(cond) is 26%. Radiative losses will be about the sameas conductive losses for the second case, indicating that a thermal timeconstant of about two hours is probably the best achievable withoutmitigation of radiation losses.

To complete the packaging of the device and preserve the thermalisolation a vacuum must be maintained in the hollow cavity 1101 aroundthe suspension plate 1100 as shown in FIG. 11 a. The suspension plate1100 has the thermal isolating structure 1102 etched into it and thenthe flexural elements and proof mass 1104. Using an abraded capacitiveplate 1106 and a backing plate 1107 sealed to the suspension plate 1100by seals 1112 under vacuum conditions a vacuum cavity 1101 is created.As part of this fabrication a gettering material such as thecommercially available “NanoGetter” film 1110 should be applied toeither the capacitive plate 1106 or the backing wafer 1107. If solderball sealing is utilized the temperature is not sufficient to activatethe standard commercial getters. Rather than use electrical resistiveheating with its requirement for additional electrical connections ifthe insulating plates are glass a laser can be shone through thematerial to local heat and activate the getter without reflowing thesolder seal. The outer cavities have thin layers of smooth reflectivemetal such as gold or aluminum deposited in unused areas as a radiativeshield 1108. The exterior of the die can also be coated with a radiationshield layer if desired.

In FIG. 11 b a design is shown in which there are two distinct cavitiesformed as described above. The inner cavity will act as a thermalreservoir for the contained sensing element 1104, while the additionalouter packaging 1114 presents an additional radiative barrier using asmooth metal deposition 1108, gettering material 1110 is present in bothcavities. Obviously this concept could be extended to additionalcavities if required for the application. This packaging concept can beused for any sensor that requires isolation from short term temperaturevariations in the environment.

During the operation of a force balance control loop using anelectro-magnetic actuator a current is required to flow in the coil tocreate the required restoring force. The process is illustrated in FIG.12 a. Here the force balance control loop 1200 outputs a restoringcurrent 1201 that flows through the coil 1206 that creates a force dueto the current's interaction with the magnetic field present in theactuator. In the seismometer there are resistive elements present in thecircuit. The resistance of the thermal isolation path is represented byresistors 1203, the resistance of the flexural elements by resistors1204 and finally the resistance of the coil itself on the proof mass bythe resistor 1205. The current passing through these resistors causes avoltage drop and heat to be dissipated in the resistors. Providing thevoltage drop does not cause the current source in the control loop tofall out of compliance this will not affect the performance of the forcebalance loop. However, the heating can cause a non-linearity in thesystem response as the restoring current causes heating and changes inthe spring rate of the flexural elements.

A technique to minimize this effect is shown in FIG. 12 b. In thisfigure an oscillator has been added to the circuit that produces afrequency considerably above the seismic band of interest and such thatthe force produced is filtered out by the mechanics of the system. Thissignal is then passed through a voltage controlled oscillator 1210resulting in an amplitude modulated signal 1212, this is injected viacapacitor 1214 into the coil circuit. The voltage of the coil drive 1216including the resistors is input to the buffer and RMS level detectorand low passed filtered 1218. This voltage 1220 is then compared to a DCvalue 1222 that sets the operating point for power dissipation in theresistors. The output of the amplifier 1223 is then used to control thevariable gain amplifier 1210. With the feedback loop properlycompensated the circuit will vary the ac signal inversely to the seismicsignal such that the RMS power dissipation is maintained constant withinthe operating range of the loop. The implementation described is justone of many possible implementations of such a control loop. The novelfeature is the use of a high frequency amplitude modulated AC carrier tomaintain constant heating in the resistors as the low frequency seismicsignal varies.

An additional input is shown as digital temperature compensation 1224,one possible implementation of this is shown in FIG. 12 c. Generally adigital system will be preferred for this system due to the very longtime constants required in the control loop. The system is illustratedwith inputs from three temperature sensors, sensor 1226 monitors theexternal temperature, sensor 1228 monitors the temperature of the frameafter the thermal isolator, while sensor 1230 monitors the temperatureof the proof mass. The temperature reading is converted to a digitalstream via the ADC 1231 and read by the microprocessor 1232. Thefirmware within the microprocessor can be written using several controlstrategies known to those skilled in the art to produce one or moreoutputs that can be converted to an analog voltage by the DAC 1234. Thevoltages can be fed into the control loop of FIG. 12 b 1224 or they canbe used to dissipate power in a resistor 1236 that would be situated onthe frame after the thermal isolator. The number of sensors and heaterlocations could be increased in this scheme to further reduce thetemperature variation seen on the springs.

FIG. 13 illustrates the spurious mode rejection ratio for in-axis andout of axis frequencies as the number of intermediate frames isincreased. We can see from FIG. 13 that in order to maximize therejection ratio for both in-axis and out of axis frequencies, the numberof frames that should be incorporated into the design is five, onebetween each of the six flexural elements. As the rejection ratio risesmore steeply for the off-axis case than it falls for the on-axis case,there will be an overall tendency for more frames to produce betterperformance.

If we take an example with more flexural elements we can calculate moredata points and see again the convergence of the “on-axis” and“off-axis” modes to give an improved overall rejection ratio. Forexample, in one preferred embodiment let us assume we have twenty-fourflexural elements in order to achieve a desired frequency response. Forthis case, let us again plot the in-axis and out-of-axis frequencies inrelation to the fundamental frequency, the so called “spurious-moderejection ratio”. FIG. 14 illustrates the spurious mode rejection ratiofor in-axis and out of axis frequencies as the number of intermediateframes is increased. We can see from FIG. 14 that in order to maximizethe rejection ratio the maximum number of frames utilized in the designshould be approximately twenty-three, one between each intermediateframe should be incorporated into the design.

It is important to note that in some designs it may be desirable forother system considerations to not optimize for an equivalent spuriousmode both for the in-axis and off-axis, but to allow say a loweroff-axis spurious mode compared with the in-axis mode. This could beused for example when the off-axis is suppressed by the DisplacementTransducer geometry, while the in-axis mode is not. The techniquespresented can be used for any desired optimization.

The invention also preferably includes a dampening structure that ishighly effective during non-powered/non-operational states (i.e. whenthe feedback control system is not powered and does not provide anydampening). Preferably, this dampening structure includes a spring/gasdampening structure configured to provide damping during non-poweredstates. FIG. 15 illustrates a perspective view of a suspension plate1500 having a spring/gas dampening structure 1510 in accordance with apreferred embodiment of the present invention.

As shown in FIG. 15, each of the intermediate frames 1501 is preferablylarger (longer) in length then the flexural elements 1503 disposedbetween each of the frames, with each frame traversing a larger portionof the internal cavity 1502. The intermediate frames are alsosufficiently rigid, but as light as possible, in order to suppress outof plane movement of the proof mass while also suppressing spuriousresonant frequencies without breaking or fracturing. The intermediateframes 1501 are designed to physically contact with each other beforethe flexural elements 1503 interspersed between them are compressedsufficiently to cause damage to the flexural elements 1503.

In order to prevent fracturing and/or damage due to extreme externalshock or vibration, the invention preferably further includes thespecially formed spring/gas dampening structure 1510, which providesadditional damping to the system during non-powered states.

Turning to FIG. 16, there is shown a close-up view of a preferredembodiment of the spring/gas dampening structure 1510. As shown, thepreferred embodiment preferably includes one or more trapezoidal shapedpistons 1601 and engagement apertures 1602. In a preferred embodiment, apiston 1601 is preferably positioned on the last (most outward)intermediate frame 1605, facing outward, and the correspondingengagement aperture 1602 is then positioned on the inner surface ofouter frame of the suspension plate 1607, facing inward. As the mostoutward intermediate frame 1605 approaches the inner surface of theouter frame of the suspension plate 1607, the piston 1601 will engageand insert into the aperture 1602, thereby providing a dampening effectbefore the intermediate frame can contact the surface of the outer frameof the suspension plate.

In a preferred embodiment, the cavity of the suspension plate ispreferably filled with a non-conductive gas such as air or nitrogen. Asthe outermost intermediate frame 1605 moves toward the inner surface ofthe outer frame of the suspension plate 1607, the piston 1601 engageswith and inserts into the engagement aperture 1602. As the pistonrecedes further into the aperture, the gas within the engagementaperture increases in pressure, causing a force to be exerted againstthe piston and slowing the motion of the intermediate frame until,possibly over multiple oscillations of the spring mass system, it comesto rest, thereby preventing damage to the flexural elements.

Alternatively, the cavity within the suspension plate may be evacuated.In this case, the spring/gas dampening structure is preferably comprisedof an aperture and a corresponding piston wherein the piston is actuallyformed of two separate portions coupled together using a smallresistance spring. FIG. 17 is a close-up view of such an alternativeembodiment of a piston 1700 used in a spring/gas damping structure,wherein the piston is formed of two separate portions coupled togetherusing a small resistance spring. As shown, the piston includes a firsthalf-portion 1701 and a second half-portion 1703, which are coupledtogether using small resistance springs 1705. In normal operation whenthe pistons are not engaged these two spring elements are separate, butas the parts contact they form a spring element. As the piston 1700inserts further into the aperture of the spring/gas dampening structure,second half portion 1703 of the piston is pushed against and closer tothe first half portion 1701 while the resistance spring provides a forceagainst the second half portion 1703. As the second half portion 1703moves closer to the first half portion 1701, the resistance from thespring increases. This spring motion can be used both to dissipateenergy, but also to act as an energy store to disengage the first andsecond half portions to prevent them “sticking” together by the force ofstiction and preventing the device from functioning as a spring masssystem. Alternatively, as shown in FIG. 18, a layer of damping materialsuch as a visco-elastic polymer 1706 may be inserted between the firsthalf portion 1701 and the second half portion 1703, in place of or inaddition to the resistance spring. A visco-elastic material block 1707can also be deposited on top of the spring element 1705 to providedamping and energy loss in the spring.

While the description above refers to particular embodiments of thepresent invention, it will be understood that many modifications may bemade without departing from the spirit thereof. The presently disclosedembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims, rather than the foregoing description,and all changes which come within the meaning and range of equivalencyof the claims are therefore intended to be embraced herein.

1. A seismometer comprising: at least one fixed capacitive plate; afirst capacitive sensor array positioned on a surface of said fixedcapacitive plate, said first capacitive sensor array having a periodicpattern of conductive elements; a suspension plate having a proof masssupported by a plurality of flexural elements capable of substantiallyconstraining motion of said proof mass to a single axis with at leastone intermediate frame positioned within and between said flexuralelements, said flexural elements being predeflected when lying flat,whereby said proof mass is centered on said suspension plate when saidsuspension plate is at a “Galperin” orientation to a vertical axis; asecond capacitive sensor array positioned on a surface of said proofmass having a periodic pattern of conductive elements aligned in acommon direction of periodicity parallel to said conductive elements insaid first capacitive sensor array in separated opposition; anelectrical connection to said second capacitive sensor array on saidproof mass allowing a coupling of cyclic excitations from externalcomponents through said periodic pattern of said second capacitivesensor array to said periodic pattern of said first capacitive sensorarray, said coupling ranging between zero and one hundred percent andbeing a cycling positional measure of said proof mass with respect tosaid fixed plate; and an electrical connection to said capacitive platesensor array transmitting a signal resulting from said coupling of saidfixed plate sensor array to said proof mass sensor array to externalelectronics, said external electronics determining the percentage ofsaid coupling between the first capacitive sensor array and the secondcapacitive sensor array in order to transduce the position of said proofmass relative to said fixed plate.
 2. The seismometer of claim 1,wherein said capacitive plate is comprised of insulating material. 3.The seismometer of claim 1, further comprising a sandwich of a fixedcapacitive plate, a proof mass plate and a capping plate, wherein thethickness and material of said fixed capacitive plate and said cappingplate are substantially identical.
 4. The seismometer of claim 2,wherein said insulating material is glass.
 5. The seismometer of claim2, further comprising a differential displacement transducer pickupcapacitor.
 6. The seismometer of claim 2, further comprising anadditional capping plate on the back of said proof mass plate, saidcapping plate forming a protective enclosure around said periodicelements on the surface of said proof mass plate, such that said fixedcapacitive plate is on one side of said proof mass plate and saidcapping plate is on the other side of said proof mass plate.
 7. Theseismometer of claim 6, wherein said capping plate comprises at leastone cavity, said cavity being formed by micro-abrasion with a protectivemask.
 8. The seismometer of claim 6, wherein said fixed capacitive platecomprises at least one channel for relieving a surface of said fixedcapacitive plate unused for said first capacitor array, said channelbeing formed by micro-abrasion with a protective mask.
 9. Theseismometer of claim 4, further comprising a high resistivity filmpositioned over said glass material to prevent static charge build up.10. The seismometer of claim 1 fabricated in a batch fabrication as awafer sandwich, whereby individual die are capable of being separatedfrom said sandwich using controlled fracture of weakened supports formedby micro-abrasion in said fixed capacitive plate and said capping plateand thin supports formed by DRIE in said proof mass plate.
 11. Theseismometer of claim 1, wherein said capacitive plate is attached tosaid suspension plate by a plurality of solder balls of essentially thesame diameter.
 12. The seismometer of claim 1, further comprising anelastomeric connector having a plurality of wires, said plurality ofwires contacting with a plurality of connection pads on said capacitiveplate and with a plurality of connection points on said externalelectronics.
 13. The seismometer of claim 12, wherein said embeddedwires are gold plated.
 14. The seismometer of claim 12, wherein saidelastomeric connector is positioned in a slot formed in said suspensionplate.
 15. The seismometer of claim 1, further comprising an additionalelectronic circuit, said electronic circuit being capable ofcompensating for variations in the heating power of said externalelectronics by injecting additional heating power into said externalelectronics.
 16. The seismometer of claim 15, wherein said additionalelectronic circuit comprises a high frequency amplitude modulatedalternating current (AC) carrier to maintain constant heating in saidexternal electronics as the low frequency seismic signal varies.
 17. Theseismometer of claim 16, further comprising a digital control loop, saidcontrol loop being capable of compensating for external temperaturevariations maintaining the temperature of said seismometer.
 18. Theseismometer of claim 1, further comprising a plurality ofinterconnecting beams etched into said suspension plate, said pluralityof interconnecting beams providing thermal isolation for said suspensionplate.
 19. The seismometer of claim 1, wherein said suspension plate isthermally isolated through vacuum sealing.
 20. A method of attaching twowafers in a seismometer, the method comprising the steps of: a) forminga patterned wetting metal layer on a surface of a first wafer; b)forming a plurality of cavities by micromachining a surface of analignment wafer, said cavities being arranged to match the pattern ofthe wetting metal layer on the surface of said first wafer; c) pouring aplurality of solder balls on the surface of said alignment wafer,thereby populating each said cavity with a solder ball; d) pouring offexcess solder balls from the surface of the alignment wafer by tiltingsaid alignment wafer; e) aligning said first wafer with said alignmentwafer and connecting the first wafer with the alignment wafer such thateach wetting metal layer of said first wafer comes into contact with amatching solder ball of said alignment wafer; f) immobilizing saidsolder balls on the wetting metal layer of said first wafer byperforming a partial reflow onto said wetting metal layer; g) removingsaid alignment wafer; h) forming a patterned wetting metal layer on asurface of a second wafer, said patterned wetting metal layer beingarranged to match the pattern of the wetting metal layer on said firstwafer; i) aligning said first wafer with said second wafer andconnecting the first wafer with the second wafer such that each solderball immobilized on said first wafer comes into contact with a matchingwetting metal layer of said second wafer; and j) bonding the first waferto the second wafer by heating and reflow of said immobilized solderballs.
 21. The method of claim 20, wherein the wetting metal layer onthe first and second wafers is formed of a plurality of metallized pads.22. The method of claim 21, wherein said solder balls have apredetermined volume of solder and said metallized pads have apredetermined pattern of metallization, thereby resulting in acontrolled thickness of separation between said first and second wafers.23. The method of claim 20, wherein the plurality of cavities in saidalignment wafer is formed by Deep Reactive Ion Etching.
 24. The methodof claim 20, further comprising the steps of forming channels in thecavities of the alignment wafer, said channels going through saidalignment wafer; and applying a vacuum force to said through channelswhen said plurality of solder balls is poured on the surface of saidalignment wafer; thereby creating suction in said cavities andattracting said solder balls into said cavities.
 25. A method ofattaching two wafers in a seismometer, the method comprising the stepsof: a) forming a patterned wetting metal layer on a surface of a firstwafer; b) forming a plurality of cavities by micromachining a surface ofsecond wafer, said cavities being arranged to match the pattern of thewetting metal layer on the surface of said first wafer; c) forming apatterned wetting metal layer on said second wafer in proximity to saidcavities; d) pouring a plurality of solder balls on the surface of saidsecond wafer, thereby populating each said cavity with a solder ball; d)pouring off excess solder balls from the surface of the alignment waferby tilting said alignment wafer; e) aligning said first wafer with saidsecond wafer and connecting the first wafer with the second wafer suchthat each wetting metal layer of said first wafer comes into contactwith a matching solder ball of said second wafer; f) bonding the firstwafer to the second wafer by heating and reflow of said solder balls.26. A method of aligning a seismometer die with an instrument mountingframe, the method comprising the steps of: a) forming a first cavity ona surface of said seismometer die capacitive plate by micro-abrasion,said first cavity being substantially circular; b) forming a secondelliptical cavity on a surface of said capacitive plate bymicro-abrasion, said second cavity having a minor diameter equal to adiameter of said first cavity; c) machining three location holessuitable for constraining balls into said instrument mounting frame in apattern to match said first and second cavities in the capacitive platesuch that said third location hole forms an apex of a triangle inrelation to said first two location holes; d) inserting a first ballinto said first cavity or first location hole; e) inserting a secondball into said second cavity or second location hole; f) positioning athird ball on the surface of said capacitive plate, whereby thecombination of the first, second, and third balls on said capacitiveplate forms a three-point mount for a seismometer die and preventsexcess stress on the die as it is mounted onto said instrument frame bycompressing said die uniformly with a resilient pad against saidinstrument frame.
 27. The method of claim 26, wherein each of said ballsis ceramic.
 28. The method of claim 26, wherein each of said balls ismetallic.
 29. A method of providing thermal insulation for a seismometerdie, the method comprising the steps of: (a) providing a backing plateand a capacitive plate; (b) selectively applying a gettering material toone or both of said plates; (c) positioning at least one strip of asmooth reflective material on a surface of one or both of said plates,said smooth reflective material acting as a radiative shield; (d)sealing said capacitive plate and said backing plate to a suspensionplate under vacuum conditions to create a first vacuum cavity; and (e)activating said gettering material using a laser.
 30. The method ofclaim 29, further comprising the step of forming a second vacuum cavityaround said first vacuum cavity, said first and second cavities havingsubstantially identical structure and being vacuum sealed.